In the prior art, formalin resin obtained by using cashew nutshell oil, drying oil, etc. or a rubber, such as a natural rubber or SBR have been used as relatively less expensive organic friction dusts for friction material. However, these prior friction dusts have poor thermal resistance, and mechanical strength upon heating, and The upper limit of use temperature thereof is low.
For example, a rubber-based dust obtained by thermal-treating carbon-containing rubber, such as used tire, followed by grinding the thus-treated rubber has a temperature, at which weight loss or heating starts, of as 150.degree.-200.degree. C. as, and 10% weight loss on heating, which is used as a thermal resistance factor, is as low as 200.degree.-230.degree. C. In addition, when the rubber is in the state of a high temperature the rubber becomes pitchy. Therefore, the friction coefficient of the rubber is lowered greatly.
Cashew resin dust which is used in the greatest amount undergoes 10% weight loss on heating in air of 260.degree.-300.degree. C., and is thermally decomposed at a temperature of 520.degree.-550.degree. C.; residual carbon of cashew resin dust upon thermal decomposition is very low. The cashew resin dust is decomposed via pitch at a temperature of about 280.degree. C. similar to rubber-based dust. Therefore, the friction coefficient thereof is lowered greatly. The dust also has great shortcomings from the viewpoint of safety. In addition, since a strongly acidic compound is used as a catalyst for preparing the cashew dust, a metallic filler is likely to be corroded by the remaining acidic compound.
Conventional friction materials which use asbestos as a reinforcement substrate and a phenol resin as a binder resin are increasingly supplanted by non-asbestos types as exemplified by semi-metallic friction materials using steel fibers as a reinforcement substrate and a friction material that uses aramid fibers or a carbon filler as a chief component of the reinforcement substrate. However, it is known that disk brake pads using those substitute reinforcement substrates have proved to generate harsh sound generally called "a squeal of brakes" when braking is applied.